THE DRIFT AND GEOLOGIC TIME EU 
of a quarter of a degree for a thousand miles would require an 
elevation at the point of origin of the flow of something approach- 
ing five miles. The probability, and we may say the certainty, is 
that the slope was not uniform and that over large distances the 
ice traveled over dead levels, and in parts even stagnated, the 
upper part flowing over the lower débris-laden portions. That 
the contained débris has a retarding influence on the flow of 
glaciers has been urged by O. P. Hay," I. C. Russell,? and R. D. 
Salisbury,3 and it appears that this may even cause absolute stag- 
nation under some circumstances. Even detached erratics seem 
to progress more slowly than the body of the ice in certain 
instances; witness the well-known observation of Professor W. 
H. Niles# on the Aletsch glacier where ice moved so much more 
rapidly than a contained bowlder as to leave a free tunnel fora 
considerable distance on its lee side. Even lighter substances 
appear to be occasionally retarded in their progress as compared 
with the ice. Recognizable remains of buried travelers have 
been taken out of Alpine glaciers even hundreds of years after 
their loss.5 It is impossible, therefore, to claim that these errat- 
ics could have traveled at the same rate as the surface of the 
glacier, and when we consider that they bear the marks of hav- 
ing been subjected to scouring in the ground moraine that has 
left their surfaces flattened and striated, the probability of such 
a rate of progression is certainly very much diminished. 
Allowing a flow of two feet per day to the ice-sheet, which is 
undoubtedly far above the real rate of the ice movement, it would 
require 7200 years for Mr. Upham’s bowlder to travel its thousand 
miles from Hudson Bay to southern Minnesota, and this with- 
out any delay from friction or attrition in the ground moraine, 
or stagnation in the lower strata of the ice. Taking, also, into 
account the fact that the northern erratics are found at all dis- 
tAm. Jour. Sci. and Arts, 3d ser.. XXXIV, 52, 1887. 
2 Jour. GEOL., III, 823-883, 1895. 
3 Jéid., Vol. 1V, 769-810, 1896. 
4 Am. Jour. Sci., 3d ser., XVI, 367, 1878. 
5 SIR HENRY HoworTtH. Geol. Mag. N. S., Decade IV, Vol. IV, 127. 
